1. Field of the Invention
This invention relates to wave guides for radio-frequency electromagnetic radiation and, more particularly, to a microwave feed horn provided with novel means for controlling the plane of polarization of microwaves passing through the feed horn.
2. Description of the Prior Art
The electromagnetic spectrum is crowded with information signals broadcast by commercial radio and television stations, police and fire units, aircraft and air traffic controllers, ocean-going ships and harbor vessels, cellular telephone stations, military satellites, etc., and in order utilize the spectrum with better efficiency a number of techniques have been adopted. For example, commercial satellite signals are normally broadcast in polarized form, the plane of polarization being either horizontal or vertical. This enables receivers to discriminate between broadcast signals on the basis of polarization as well as frequency and therefore doubles the effective capacity of the spectrum. Of course, in order to be able to receive signals of both polarizations, the receiving apparatus must include a device that is adjustable so that it is capable of transmitting signals of either polarization. Signals having the polarization not selected by the device are reflected and are therefore not demodulated by the receiving apparatus.
A patent to Moeller et al. No. 3,024,463 discloses a feed mechanism by means of which microwave signals may be transmitted between a feed horn of circular cross section and a wave guide of rectangular cross section. The mechanism is capable of transmitting signals that are either linearly or circularly polarized. In the case of either polarization of radiation received by the feed horn, the radiation is linearly polarized at the rectangular wave guide.
A patent to Bleackley No. 3,296,558 discloses a wave guide for rotating the plane of polarization of a wave. The apparatus comprises metal rods mounted on a torsion wire and extending out from the wire. If the wires are caused to define a surface that twists along the length of the wire, the plane of polarization of electromagnetic radiation propagated through the wave guide can be rotated.
A patent to Raiman No. 4,503,379 discloses another form of probe for rotating the plane of polarization of microwave radiation propagated through a circular wave guide. The probe comprises a continuous, serpentine, electrically-conductive filament. The upstream end of the filament is rotatable and the downstream end is fixed. The rotatable end of the filament is adjusted to receive microwave radiation of a particular polarization. The probe rotates the plane of polarization of the transmitted radiation so that, at the downstream end of the probe, the plane of polarization of the propagated radiation is such that the radiation will pass through a rectangular wave guide coupled to the downstream end of the probe.
The means known heretofore for rotating the plane of polarization of a radio frequency electromagnetic wave propagated through a wave guide all suffer from certain drawbacks. A particular problem is that the means for rotating the leading end of the probe tends to be complicated, expensive to manufacture, slow in operation, prone to break down, and expensive to repair. In commercial apparatus for receiving and discriminating between electromagnetic radiation of different polarizations, it is important to be able to shift easily and reliably from reception of radiation having a first plane of polarization to reception of radiation having a second plane of polarization which is orthogonal to the first plane. It is also important that the apparatus be inexpensive, reliable, and easily repaired. Apparatus that is fully responsive to these commercial requirements has not yet been made available.